A large quantity of fine particles (particulate) mainly comprising carbon are contained in the exhaust gas of a diesel engine. Since the environmental pollution is caused if particulate is discharged into the air, there exists the technique for removing the particulate prior to the discharge of the exhaust gas into the air by installing the particulate trap device including a filter for trapping the particulate in the exhaust system of the diesel engine.
The trapped particulate is gradually accumulated in the filter upon use of the particulate trap device. Therefore, the filter function is regenerated by backwash or combustion of the filter to remove the particulate when a certain quantities of the particulate has been deposited during the use of the particulate trap device, in order to prevent engine performance from decreasing by increased exhaust resistance due to increased quantity of the deposited particulate in the filter.
However, the overall system turns out to be complex and large size in the method for removing the particulate by backwash. When the particulate is removed by combustion, on the other hand, a part of the particulate is left behind as ash components after regeneration of the filter by combustion, and this residue is accumulated in the filter after many times of regenerative combustion. Consequently, there is such a problem that the filter is lost by being molten at a temperature lower than the melting point of the filter material due to the reaction of this residue with the material constituting the filter at an elevated temperature during operation of the device or in the regenerative combustion treatment.
Actually, the deposited particulate contains elements such as P, S, Ca, Na, Zn, Cr, Fe, Ni and Cu originating from fuels, engine oils and pipe and tube parts. melting as described above is caused because compounds and/or composites containing these elements are left behind after combustion of the particulate. The filters comprising oxide ceramic based materials involve severe problems since they are particularly reactive with the ash component.
Under these situations, JP-A-10-33923 discloses a technology as a method for removing deposited ash component in which the ash component is once deposited on a ceramic particle layer adhered on the surface of the filter, and they are blown off thereafter by backwash together with the ceramic particles. JP-A-11-210440 discloses a technology for selective discharge of the ash component by constituting the seal at the outlet side of DPF with a mesh member.
Other technologies for retaining a catalyst on the filter for facilitating combustion of the particulate have been investigated independently from the technologies described above. For example, JP-A-2-75314 discloses a method for immersing DPF in a mixed solvent of silica sol and catalyst solution.
JP-A-57-91726 discloses a technology in which an inorganic carrier is allowed to contain a molybdic acid salt or a mixed acid of molybdenum oxide and an metal oxide to obtain an exhaust gas filter of an internal combustion engine. JP-A-2-60374 discloses a technology in which a tungstic acid salt and a platinum group element are allowed to be dispersed and loaded on a porous inorganic substrate carried on a refractory structure of the three dimension having a gas filter function. JP-A-10-274030 discloses a technology in which a metallic component having a catalytic activity is retained on a porous filter of an auto-pyrolitic type refractory metal via a catalyst carrier comprising a heat-resistant inorganic oxide.
However, it is a problem for the technology disclosed in JP-A-10-33923 that the overall system becomes complex and large size as in the foregoing backwash component regeneration method due to forced peeling of the ceramic particles on which ash components have been adhered. The technology disclosed in JP-A-11-210440 involves such a problem that it is difficult to permit only the ash component to pass through the outlet side mesh member selectively from the particulate and the ash component having different particle size distribution with each other, thereby either trap efficiency of the particulate or discharge efficiency of the ash component is forced to be sacrificed.
Although Japanese JP-A-2-75314, 57-91726 and 10-274030, and JP-B-2-60374 have proposed technologies in which catalysts are designed for complete combustion of the deposited particulate at a lower temperature, no idea or investigation from the view point of suppression of melting of the filter due to the deposited ash component, as well as reactivity or interaction between the loaded catalyst and deposited ash component, has not been investigated at all.
After all, technologies that can completely solve the problem “The filter is finally lost by being molten at a temperature lower than the melting point of filter.” have not been found yet. Such problem arises as a result of accumulation of a part of materials captured by the ceramic filter remained as ash component after regenerative combustion in the filter by many times of regenerative treatments by combustion or long term continuous regeneration, followed by a reaction of the ash component with the filter component at an elevated temperature during use or during the regenerative treatment by combustion in the system for regenerating filter functions. Such system uses a high temperature ceramic filter comprising a material represented by a oxide based ceramic such as cordierite, carbide such as SiC and/or a carbon nitride such as Si3N4, and regenerates the filter function by removing materials captured by the filter by, for example, continuous combustion or intermittent combustion after deposition of a certain amount of the materials captured by the filter.
The present invention has been made, in view of the above-mentioned situations, and, therefore, the object thereof is to provide a ceramic filter and a filter device capable of using for a long-term, by suppressing melting of the filter derived from the reaction between the ash component contained in the materials captured by the filter which are captured by the ceramic filter and remains accumulated by being not removed by a high temperature treatment such as combustion of the filter, and the filter component at a temperature lower than the melting point of the filter component.